A servo system is a control system that controls the operation of an apparatus through a series of feedback signals to the control system. The feedback signal is typically from a sensor that determines the offset of the actual operation of the apparatus from the requested operation of the apparatus. Some examples of servo systems are: a thermostat for controlling the temperature of a room; an autopilot for controlling the flying attitude and direction of an aircraft; a cruise control for controlling the speed of an automobile; a servo system in a hard disk drive (HDD) for controlling disk RPM and magnetic transducer position.
These examples of servo systems are challenged by external disturbances that can cause them to over or under react in the control of the operation of the apparatus. Examples of external disturbances, and over and under reaction they may cause are: the air conditioning of a room may not be adjusted quickly or may cool a room too much in response to changing thermal loads; an aircraft may not respond smoothly or accurately to wind or barometric conditions to maintain an attitude or direction of the aircraft; an HDD may not react in a timely manner to demands for data from the HDD.
An HDD is one example of many possible examples of a mechanism using a servo system. One of ordinary skill in the art will appreciate that embodiments of the present invention are beneficial to a variety of mechanisms operable to a control or servo system. The HDD will be used for the sake of brevity and clarity to demonstrate the need for a well-controlled servo system and a servo system's vulnerability to problems.
Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating data and for holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the HDD, has undergone many changes.
The amount of data that can be stored on a disk is governed by many well-known physical principles. One factor in determining the amount of data that can be stored in an HDD is the ability of the magnetic transducer to write closely spaced data tracks onto the disk surface. The spacing of tracks on a disk surface is known as track pitch, and the unit of measure for expressing the density of tracks on a disk is tracks per inch or TPI. A smaller track pitch results in a higher TPI. The servo system of the HDD enables in part the magnetic transducer to be precisely positioned at a radius on the disk to write a data track, enabling adjacent data tracks to be written with a high TPI.
Once the data track is written, the HDD must be able to find the track and settle upon it in a minimal amount of time. This amount of time is typically known as seek-settle time, and when coupled with the time for the data to rotate to where it can be read and the time required for the HDD to decipher the data on the track, this total time is known as latency. An HDD user usually prefers short latency. The robustness of the servo system will determine if the magnetic transducer has settled on a desired data track or needs to make another attempt at settling on the data track. Spurious electrical and/or mechanical noise can degrade the servo system and require it to make multiple attempts at reading data. The susceptibility of a servo system to spurious noise may prevent the magnetic transducer from being positioned in a sufficiently precise manner to allow a desired TPI.